Currently, some structures are industrially manufactured without foaming, for example, crates for bottled beverages, which then have foam integrated into existing cavities in order to reduce the weight of the structure, compared to a solid structure, and improve its impact resistance, without damaging the functionality, stack-ability and mechanical stability.
For said process of filling structures with foam, the steps defined in WO2010008264 are followed. This document refers to a process for producing molded plastic articles with thickened and reinforced walls, where the process combines conventional plastic molding techniques and comprises the steps of: designing a plastic article with at least one cavity or hollow area to be filled with a thermoplastic reinforcing material and which should have at least one injection gate; pre-molding the plastic article using a conventional molding process; injecting a thermoplastic material with a foaming agent through the injection gate using a low pressure injection machine; removing the product or plastic article from the low pressure injection machine; and, cooling the manufactured product in a storage area.
Following the steps indicated in the document WO2010008264, a High Density Polyethylene (HDPE) is used, with a melt flow index (MFI) of 8 grams per 10 minutes at a temperature of 190° C. with a weight of 2.16 kg which is injection molded. After manufacturing the un-foamed structure, a foamed polymer material, Linear Low Density Polyethylene (LLDPE), with a MFI of approximately 65 g/10 min (190/2.16), is injected into the structure. The injection molding processing temperature of the foamed material is approximately 150° C. The foam is generated with an endothermic chemical foaming agent, commercially available as Microcell® 303 from Momentum International GmbH.
The process defined in the document WO2010008264, features the problem that the injection of the foamed material must be done at an elevated temperature (more than 130° C. for the LLDPE) which liberates the residual stresses in the un-foamed structure, deforming said structure, which is an undesirable effect because it compromises the stack-ability, the mechanical stability, and the functionality of said structure. This problem is currently addressed by modifying the injection mold geometry to compensate for the deformations. However, this solution is inadequate, since it implies a process of trial and error of offsetting the deformations of the final structure to the mold cavity shape, increasing cost and time of development.
Currently, there exist many known processes and methods in the state of the art for the foaming of a polymer and its later application within another polymeric material.
One such solution in the state of the art is filling the cavities of the structure with chemical components in order to obtain thermoset foam that reacts at a temperature sufficiently low such that deformations in the structure are not produced. The document U.S. Pat. No. 3,389,824A discloses an example of the use of this solution for the construction of a cooler utilizing polyurethane as the foamed material. This method is commonly used in current fabrication techniques, but it creates enormous difficulties with the recycling of the polymeric structure because the thermoset foam cannot be melted. Other examples are presented in the documents U.S. Pat. Nos. 6,093,358 and 6,295,787 where a thermoset expandable material is used to fill the cavities of a plastic part.
Another solution available in the state of the art is integrating the foamed material with the un-foamed structure in the same mold wherein the structure is manufactured, said process being known in the art as foam overmolding. An example of this solution is disclosed in the document EP 2318282 A1, in which a rigid preform is overmolded with polymeric foam to later obtain, through a blow molding process, a container with a foamed layer. The pressurized mixing of the melted polymer to be foamed with a gas in the supercritical state is a technique commonly used to obtain thermoplastic foams that can be used to fabricate mono-component or overmolded multicomponent structures. Said mixture generates the foam when it is submitted to a low pressure condition. One example of this foaming technology is presented in the document US 20100198133. The principle disadvantage of the use of these overmolding and foaming technologies is the high investment cost in processing equipment and in mold technology.
Another solution available in the state of the art is the filling of part cavities with expandable polymeric beads. An example of this solution is described in the document US 20140110491, where a plastic structural article cavity is filled with a steam expandable thermoplastic polymer beads, when they are expanded, the cavity is filled. The materials of the beads and the structural article are of a similar polymer, enabling the recycling. However, the adhesion between the materials may be compromised, constraining the invention to closed cavity geometries. A similar solution is presented in the document EP 0647513. Another example is presented in the document U.S. Pat. No. 5,665,285, where a molded foam article is integrating with a polymeric skin using the blow molding method and expandable beads for foaming. This solution requires that the expandable beads are introduced in the hollow cavity prior to cooling.
One of the steps of the invention described in the present document is exposing the polymer which will be foamed in its solid state to a high pressure gas. This technique has been a part of various disclosures, but they are not intended for the low temperature integration of polymeric foam with polymeric bodies for obtaining a final structure with improved properties, without deforming the polymeric body, in order to guarantee the functionality and other properties of the final structure.
One of said disclosures is the document U.S. Pat. No. 7,107,601 which divulges a method for manufacturing an anti-vibration device which comprises the steps of: saturating a resin material with an inert gas through adjusting the pressure and the quantity of inert gas; molding a product in which the number, form, and shape of the gas cells are adjusted by controlling the injection pressure, injection velocity, shot size, holding pressure, cooling gradient and the cooling time.
Another document related to this technology is U.S. Pat. No. 7,182,897 which teaches a method for storing a material after it has been saturated, wherein the material is saturated at a pressure not less than 4 MPa and a defined temperature. Depending on the type of material, the time, the pressure and the saturation temperatures, the storage conditions are defined.
The document EP0765724 discloses a method for extruding plastic foams while reducing the viscosity by means of a gas. In this process, the material in granule or powder form is fed to a gas absorption apparatus, where it is charged with gas under a defined pressure and temperature and later passes through an extrusion process.
The state of the art is plenty of documents that describe the contact of polymer pellets with a gas in order to produce bead foams. For example, Li et al use a high pressure vessel to impregnate iPP polymer with N2 and CO2. The vessel is heated in order to foam the material. Chen et al. use a foaming chamber to prepare EVA foam samples using CO2 as blowing agent. When the material is heated in the vessel and the pressure is released, the foaming process occurs.
The state of the art reports several studies of foam morphology and mechanical properties using injection molding to produce the foamed sampled test, where the polymer pellets were previously impregnated with a physical agent. An example is presented by Florez, where polycarbonate pellets are placed in contact with CO2 at high pressures for more than 20 hours, then, the material is injected to obtain the samples.
Finally, the PCT application WO2006100517 discloses a process for introducing a gas into a polymer which comprises the steps of exposing a first polymer to a gas at a temperature greater than the room temperature, where this step is carried out at a temperature from the glass transition temperature to the crystal melting temperature for a semicrystalline material or below the glass transition temperature for an amorphous material. The polymer is melted to produce a foamed article.
In accordance with the previous information, it is clear to those skilled in the art that the existing documents do not offer an adequate solution to the problem raised in the present invention, since in most of the cases, the process for integrating the foam is carried out at a temperature greater than the temperature at which residual stresses are liberated in the un-foamed polymeric body, which in turn causes the final structure to be deformed and compromised. In other cases, the recyclability is negatively affected or large investments in equipment and mold technology are required.
Thus, there exists a need in the state of the art for designing a process or method for integrating a polymeric foam with an un-foamed polymeric structure at low temperatures to obtain a final structure, which is to say, at temperatures lower than the temperature at which residual stresses are liberated in the un-foamed polymeric structure, with the object of maintaining the physical properties of said structure and being suitable for multiple applications. Additionally, a process that does not require large investments in equipment and mold technology and does not affect the recyclability of the final structure is needed for future applications.